Concatenating string using for loop makes it begin with 'undefined'? (javascript)

Where is undefined coming from? I thought I initialised the string correctly. (tried using debugger but can't figure out the issue)

I've tried using concat() method. I've tried +=. Everything results in "undefined<reversed string>"

Why?

const reverseString = function(string) {
    let newWord = "";
    for (i = string.length; i > -1; i--) {
        newWord = newWord.concat(string[i]);
    }
    return newWord
};

Edit: in case it's unclear when running console.log(reverseString("Hello"))

expected output would be "olleH"

actual output is "undefinedolleH"

As @VLAZ mentioned in the comment, your string.length is more than the actual length of the string that's why it bounces out of the array.

There are ways you can get rid of the undefined on your string. One is what VLAZ said which is just to -1 the length. The other one is using a Nullish Coalescing Operator on your concatenation. Samples below

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const reverseString1 = function(string) {
    let newWord = "";
    for (i = string.length - 1; i > -1; i--) {
        newWord = newWord.concat(string[i]);
    }
    return newWord
};

const reverseString2 = function(string) {
    let newWord = "";
    for (i = string.length; i > -1; i--) {
        newWord = newWord.concat(string[i] ?? "");
    }
    return newWord
};

console.log(reverseString1("Hello1"));
console.log(reverseString2("Hello2"));

<!-- end snippet -->

Index should start with (n-1), not from n

const reverseString = function(string) {
    let newWord = "";
    for (i = string.length - 1; i > -1; i--) {
        newWord = newWord.concat(string[i]);
    }
    return newWord
};

const reverseString = function (string){
return string.split('').reverse().join(''))};
console.log(reverseString("abc")); // "cba"

String (and Array) Methods are a great way to simplify this task.
Check out String.prototype.includes(), String.prototype.replace(), String.prototype.split() and more on MDN.